This invention relates to servicing of offshore wells, and particularly to servicing of wells having subsea blowout preventer stacks.
The use of subsea blowout preventer stacks during drilling and completion of offshore wells is common. Typically, such wells are lined with one or more strings of casing as drilling of the well progresses, and the casing is cemented in place by injection of cement into the annulus between the casing string and the borehole wall. Prior to completing the well, particularly in cases where the well penetrates more than one potential producing formation, it is important to evaluate the cement job to determine if multiple formations are isolated by the casing cement. If there are voids in the cement column then undesirable flow communication between formations can occur.
Cement jobs are evaluated by running an acoustic pulse transmitter logging tool through the wellbore to determine whether the annulus between the casing and the borehole wall is full of cement. If not, the casing may have to be perforated adjacent the void areas and the voids filled by squeezing cement through the perforations. Such operations are routine, but nevertheless are costly.
When cement in a wellbore annulus sets, it often generates sufficient heat to slightly expand the casing, which on cooling contracts away from the cement to create a "microannulus". This microannulus does not normally cause production problems, but it can cause a false indication of a cement job failure due to poor acoustic coupling between the cement and casing during the cement evaluation logging procedure.
In servicing an onshore well, it is possible to pressurize the wellbore sufficiently to eliminate the microannulus and provide good acoustic coupling during the logging step. Typically a "lubricator" is attached to the blowout preventer and a wireline connected to a logging tool is lowered from the lubricator through the wellbore. The lubricator enables the well to be pressurized to eliminate any microannulus that otherwise would interfere with the logging.
Unfortunately, in subsea completions where a riser extends from a subsea blowout preventer to a work area on a fixed or floating offshore platform, it is not feasible to use a lubricator and to pressurize the wellbore using techniques suitable for onshore wells.
In some subsea completion cases, a cement evaluation log is simply run on the unpressurized well, and if a void area is indicated, then a cement squeeze is performed, even though the void indication may actually have resulted from a microannulus rather than from an actual void. This results in unnecessary squeeze jobs, with resulting costs and lost production. There has been a continuing need for an improved procedure for running cement evaluation logs in offshore wells having subsea blowout preventers.